1. Field of the Invention
The invention relates to polymerizable mixtures and optically anisotropic polymers which can be prepared therefrom.
2. Description of the Related Art
Low molecular weight nematic or smectic compounds can be readily oriented in thin layers in the temperature range of the liquid crystalline phase and then have an optical anisotropy by means of which polarized light can be influenced in a controlled manner. Technically, this effect is utilized, for example, in liquid crystal displays (LCDs), in which the brightness of individual pixels is changed by switching the orientation of the molecules by applying an electric field. By a suitable choice of compounds with high refractive index anisotropy Δn, optical delays of the order of magnitude of the wavelengths of visible light can be achieved in the case of layer thicknesses of a few μm. The optical anisotropy of these layers of low molecular weight liquid crystalline compounds is, however, stable only in a very limited temperature range, owing to the thermal motion of the molecules and the limited liquid crystalline phase region. In order to produce permanent layers having a defined optical anisotropy, which is also retained in the case of a temperature change, liquid crystalline compounds having polymerizable groups which are crosslinked by chemical reaction to give a polymer film are therefore used. Liquid crystalline side-chain polymers which are prepared from polymerizable nematic and smectic monomers and oligomers, as described, for example, in WO 96/25470, have proven particularly useful for this purpose.
For many applications, the degree of optical anisotropy Δn is of key importance. Whereas, in the case of a pure retarder function, the total optical delay Δn·d can be established not only by the material parameter Δn but also by the thickness d of a layer, this is no longer possible, for example, for the width Δλ of a cholesteric reflection band since this is directly dependent on the birefringence Δn, because Δλ/λ=Δn/n (where λ=middle wavelength of the cholesteric reflection band, n=mean refractive index of the material). The Δn of the material can be varied within certain limits by a suitable choice of the chemical groups of the mesogens, but this choice is greatly limited from economic points of view. In particular, compounds having a high Δn, such as, for example, tolanes, require complicated syntheses and, owing to the excessively high costs, are scarcely suitable for applications requiring a large amount of material.
The optical anisotropy of a liquid crystal layer is determined both by the polarizability of the individual molecules in the frequency range of visible light and by the temperature-dependent order parameter of the ensemble of all molecules (W. Maier, A. Saupe, Z. Naturforsch., Part A 14, 882 (1959); ibid. 15, 287 (1960)). In the polymerization of the liquid crystal layer, a change of optical anisotropy can occur. Since the chemical structures of the mesogens scarcely differ before and after the polymerization, it may be assumed that the optical polarizability is also similar in both cases. The order parameter is mainly responsible for the change in the optical anisotropy. In order to avoid a reduction of the order parameter during polymerization, compounds in which the polymerizable groups are decoupled from the mesogenic backbone of the molecule by an alkyl spacer are therefore generally used in the liquid crystalline mixtures known to date for the production of optically anisotropic polymer films.
Particularly suitable liquid crystals are the mixtures which are described in WO 96/25470 and contain liquid crystalline monomers or oligomers having exactly one polymerizable group and which, for increasing the crosslinking density and hence the stability of the film, have additional components which carry at least two polymerizable groups. The polymerizable groups are generally not mesogenic and disturb the order of the liquid crystalline phase. In such mixtures, liquid crystalline compounds having only one polymerizable group are therefore preferable, to those having two polymerizable groups if the object is a polymer film having as high an optical anisotropy as possible.
For the polymer films, it must furthermore be ensured that the optical anisotropy of the oriented but still unpolymerized liquid crystalline mixture is retained even after the polymerization reaction. However, it is found that, in contrast to earlier investigations with liquid crystalline monomers which carry two polymerizable groups (D. J. Broer, G. N. Mol, Makromol. Chem. 192, 59 (1991)), the polymer films produced according to WO 96/25470 often have a lower optical anisotropy than the oriented layers of the same material before the polymerization reaction.
The prior art discloses polymerizable liquid crystalline mixtures which contain nonpolymerizable liquid crystalline components, which however do not give a homogeneous polymer film after the polymerization. In particular, these films are unsuitable or suitable only to a limited extent for applications for which a high Δn is required. EP 451905 A describes anisotropic gels which are prepared from mixtures of polymerizable components with low molecular weight, liquid crystalline materials.
Characteristic of these gels is that the low molecular weight, liquid crystalline material forms a continuous phase around the polymer network. In these polymer-stabilized liquid crystalline gels, the low molecular weight, liquid crystalline components are still mobile and can be switched by the action of an external force, for example an electric field. However, they have no thermal and mechanical stability and are therefore not suitable as a stable optically anisotropic polymer film.
U.S. Pat. No. 6,181,395 B1 describes a film which is used as a broad-band circular polarizer. By segregation of an unpolymerizable liquid crystalline component in a polymerizable cholesteric material, it is intended to produce a nonlinear gradient of the composition perpendicular to the film surface, i.e. the films are not homogeneous. Nothing is stated concerning the mechanical stability and the optical anisotropy of the films. Apart from the given specific examples, no generally valid method is given as to how the LC material has to be selected in order to achieve the effect of a nonlinear pitch variation of the cholesteric reflection band, described in U.S. Pat. No. 6,181,395 B1. The resulting considerable broadening of the cholesteric reflection band is achieved independently of the value Δλ/λ=Δn/n theoretically predetermined by the birefringence Δn. The LC mixtures used in the examples contain no aromatic double ring structures in the polymer moiety and are not suitable for ensuring a high optical anisotropy in the context of this invention in the polymer films.